1. Field of the Invention
The present invention is generally related to an apparatus generally used in communication technologies. More particularly, the present invention of the apparatus relates to radio frequency identification (RFID) reader.
2. Description of the Related Art
In communication system, there is a serious problem due to improper transmitted data waveform called DC wandering. It usually happens when there is a continuous transmission of high or low signals. There have been a number of coding schemes designed to overcome this problem, for example, Manchester coding. This coding, however, has less bandwidth compare to Non Return to Zero (NRZ) coding scheme. The present invention is regard to overcome the DC wandering problems and a new reader circuit has been invented.
FIG. 1 is an example of the prior art that comprises a first resistor R1, a second resistor R2, a third resistor R3, a first capacitor C1, a second capacitor C2, a diode D1, an amplifier 30, and a comparator 40.
The first terminal of the first capacitor C1 is to receive an input voltage V1, the first terminal of the second capacitor C2 and the first terminal of the first resistor R1 are connected at a node 11. The second terminal of the first capacitor C1 and the second terminal of the first resistor R1 couple to the ground. The second terminal of the second capacitor C2 and the first terminal of the third resistor R3 couple to a negative input terminal of the operational amplifier 30 at a node 13. The reference voltage V2 is input to a positive input terminal of the operational amplifier 30.
The first resistor R1 and the first capacitor C1 form a first order low pass filter. The second resistor R2 and the second capacitor C2 form a high pass filter. The second terminal of the third resistor R3 and the output of the operational amplifier 30 couple to the input terminal of the comparator 40 at the node 14.
FIG. 2 shows the simulation result of the prior art. Referring to FIG. 2, the waveform in the first row shows an input voltage V1. The second row shows the input terminal of the comparator 40 at the node 14. It also shows a hysterias upper bound V7 and a lower bound V6. The third row shows voltage of the output terminal of the comparator 40 at the node 15. Referring to FIG. 2, the voltage at the node 14 is higher than the lower bound V6 of comparator 40 when time is greater than 122 microseconds. As a result, the voltage at the node 15 has no state change after time is greater than 122 microseconds. This error comes from the DC wandering. It has great effects on the output qualities of a communication receiver.